Magnetic field sensors are used as detector components for measuring electrical currents. Detailed information on such devices can be found in the technical literature, and reference may be made, for example, to Siemens' data book entitled “Galvanomagnetic devices” published 1976/1977, to Honeywell's Application Note “HALL EFFECT SENSING AND APPLICATION”, as well as to Allegro's Application Note STP98-1-AN.
Widely used are two different techniques for performing such measurements.
The first technique, known as open loop technique, essentially comprises a magnetic circuit including a gap in which the magnetic field sensor is inserted. The magnetic circuit is excited by current flowing in a winding, thus establishing a magnetic field of induction value B:B=KCNMIM  (1)
KC is the core constant, mostly depending on the airgap length and cross-section; IM is the current to be measured; NM is the number of turns in the excitation winding.
Although the open loop technique is very simple to implement, it provides no compensation for drifts with temperature and aging of the electrical characteristics of the magnetic field sensor utilized. Its application is therefore limited to low cost, low accuracy, applications.
The second technique, known as closed loop technique, relies on an auxiliary winding, in addition to the main excitation winding, in which a negative feedback circuit controls the current in such a manner as to obtain a zero airgap B field value. The main advantage of tracking a B=0 condition consists in a greatly reduced sensitivity to drifts in the gain of the magnetic field sensor, thus significantly improving overall accuracy.
The negative feedback circuit must be designed in such a manner as to have a passband which goes from zero frequency (corresponding to D.C.) up to the desired frequency (for A.C.). Although an apparatus for implementing the closed loop technique behaves in a manner which is very close to ideal, practical realization of the above-mentioned negative feedback circuit is relatively complex because of the wide bandwidth that typically needs to be provided. Further, if high current values are involved, it is necessary for the auxiliary compensation winding to include a large number of turns in order to reduce the required compensation current, i.e. to reduce the power required for compensation purposes.
For the measurement of magnetic fields, widely used are Hall effect probes and Magnetoresistive probes. Hall effect probes generate an output voltage, VH (Volts), proportional to the value of the biasing current, IB (Amperes), and to the magnetic field, B (Tesla), through the factor of proportionality, KH (sensitivity, with units V/AT):VH=KHIBB  (2)
The method and apparatuses described in the following description make use of Hall probes for the measurement of B field values. However, the use of magnetoresitive type of sensors is also possible. Most commercially available magnetoresitive sensors are of the Wheatstone bridge type, generating an output voltage, VM (mV), proportional to the value of the biasing voltage, VB (Volts), and of the magnetic field, B (Tesla), through the factor of proportionality, KM (sensitivity, with units (mV/V)/(kA/m)):VM=KMVBB  (3)
It shall be remarked that at the present state of the art most commercially available magnetoresitive bridge sensors are optimised for the measurement of relatively low B field values (few tens of mT), whereas Hall effect probes are better at the measurement of higher B field values (few hundreds mT). As the magnetic circuit configurations herewith described can easily achieve airgap B field values of few hundreds mT, and hence decreasing the relative importance of inaccuracies introduced by external stray fields, preference will be given to the use of Hall effect probes. In case it might turn out that for some applications also magnetorestive sensors becomes interesting, or that suitable magnetoresistive sensors might become commercially available, those skilled in the art can then easily imagine obvious ways to replace said Hall probes with said magnetoresistive sensors.
In the following, magnetic field sensors generating an output signal of the type described by equation 2 will be referred to as of the type “ratiometric with respect to the supply current”, while those generating an output signal of the type described by equation 3 will be referred to as “ratiometric with respect to the supply voltage”. It is worth reminding that although the basic Hall effect probe is of the type ratiometric with respect to the supply current, several integrated circuits are commercially available which include an on-chip Hall effect probe cell with added signal conditioning and processing electronics, effectively resulting in an magnetic field sensor of the type ratiometric with respect to the supply voltage.